The subject invention relates generally to locating a reference point and, more particularly, to establishing reference coordinates for the location of a dried droplet of liquid on a semiconductor wafer for assessing the purity of the wafer surface.
Integrated circuits are solid state devices in which electrical components and electrical connections between components are incorporated into a solid matrix by the strategic placement of various conducting, semiconducting and insulating materials. The development of the integrated circuit has led to the miniaturization of electronics by providing a strong matrix to support and protect fragile miniaturized components and connections and by facilitating the placement of the electrical components in close proximity. The integrated circuit has further served to increase the reliability of electronic devices by the elimination of moving parts and fragile electrical wiring and connections.
Integrated circuits are typically mass produced by forming hundreds of circuits called "dice" on a semiconductor substrate known as a "wafer". The circuits are formed by depositing a series of individual layers of predetermined materials on the wafer. The individual layers of the integrated circuit are produced by a series of manufacturing steps. The precise characteristics of the layers, such as composition, thickness, and surface quality uniquely determine the electronic properties and the performance of the integrated circuit.
Successful fabrication of large scale integrated silicon circuits is dependent upon the purity of the wafer surfaces. The presence of metal contaminates such as iron, nickel, copper and zinc on the semiconductor surfaces can lead to defects during high temperature processing of the wafers because those contaminates can diffuse into the semiconductor material. In an effort to monitor the purity of the semiconductor surfaces, highly sensitive methods for analyzing surface purity have been developed.
One method for determining the concentration of contamination on a silicon wafer is known as vapor phase decomposition or "VPD". Such method involves supporting the wafer in a specially designed device known as a VPD reactor. A VPD reactor typically contains an acid-resistant beaker and platforms for supporting wafers within a chamber. The wafer platforms are typically water-cooled and the beakers are generally heated by a heat exchanger arrangement integrated into the bottom of the reactor.
After the wafers are placed onto the wafer platform, a solution of hydrofluoric acid is added to a beaker. The hydrofluoric acid is heated within the beaker which causes it to vaporize within the chamber. The acid vapor permeates the chamber and dissolves the silicon oxide surface layer on the wafer. The wafer is cooled by the support platform which causes a thin film of moisture to condense on the wafer surface. The impurities on the surface of the wafer are contained in the moisture film.
The wafer is then removed from the reactor and a predetermined amount of deionized water is then immediately pipetted on to the surface. This droplet is then moved over the entire surface area by gently tilting the wafer. During this process, the impurities are collected in the droplet. The droplet is then moved generally to the center of the wafer and the wafer is placed into a vacuum chamber wherein the droplet is dried on the wafer. After the droplet has been dried, its contents are analyzed by a process known as total reflection X-ray spectrometry ("TXRF"). The principle of TXRF is based on the detection of trace elements on the surface of an optically flat substrate by their characteristic X-rays excited by an incoming X-ray beam in total external reflection geometry. Through use of this process, the presence of iron, nickel, copper and zinc on the wafer surface can be detected.
In the past, the location of the droplet of water on the wafer was approximated. Because the exact location of the droplet was unknown, accurate positioning of the TXRF machine relative to the water droplet was time consuming. Moreover, although some TXRF machines are capable of accepting coordinates of a point of interest relative to the machine head, prior methods lacked the capability of generating such coordinates for use by the X-ray machine.
Thus, there is a need for an apparatus and method for generating reference coordinates for a position on a component such as a wafer that can be used to accurately orient a device such as an X-ray machine relative to the position.
There is a further need for an apparatus having the above-mentioned attributes that is relatively inexpensive to manufacture.
There is yet another need for an apparatus having the above-mentioned attributes that is relatively portable and easy to use.